Whether and how the U.S. should encourage charitable giving has recently become a matter of public debate. The Senate Finance Committee, which is taking a fresh look at the federal tax code, earlier this month released a list of questions that it is considering, among which are proposals to repeal the charitable deduction outright or cap its value. It is clear that the charitable tax deduction—on the books since 1917—is not sacrosanct.

At the same time, questions have been raised (notably by Ken Stern, the former head of National Public Radio, in his book With Charity for All: Why Charities Are Failing and a Better Way to Give) as to whether Americas wealthiest—even with tax incentives—give as much as they should to charitable causes, and whether the causes they do support are worthy ones that help a broad range of Americans.

So this is a good time to examine the philanthropic habits of a small but particularly interesting group: the billionaires included in the Forbes 400. Do the wealthiest Americans actually give much to charity? Research and analysis by my Manhattan Institute colleague Claire Rogers provides some revealing answers based on publicly available data.

As a group, the Forbes 400 have pledged to give more to charity than the U.S. as a whole gives in a year. The 400 have pledged $328.7 billion. (The figure is derived by multiplying the net worth of the groups total membership by the percentage pledged to charity.) By comparison, total U.S. charitable giving annually is about $300 billion. In other words, the philanthropic promises of just 400 people equal what might be called the annual philanthropic GDP of the whole country. Whats more, the 400 richest Americans have already donated $96.3 billion—an average of 5.3 percent of their total net worth. That exceeds the average percentage of annual income donated to charity by taxpayers who itemize their deductions—and it exceeds the percentage of income donated to charity by President Obama before he became a presidential candidate (he donated 4.6 percent in 2005, 1.2 percent in 2004, and 0.4 percent in 2002; he has given at a much higher rate since entering the White House—21.8 percent in 2011. Vice President Biden and his wife, Jill, donated 1.5 percent of their income in 2011).

The typical (median) member of the Forbes 400 has donated 1.1 percent of net worth to charitable causes. Thats lower than the 5.3 percent average, which reflects the fact that some billionaires have already made extremely generous donations at the top end. Similarly, 93 of the super-wealthy have signed the Giving Pledge initiated by Bill Gates and Warren Buffet and pledged to give away at least half of their wealth over the course of their lifetimes (or shortly after their death). Its worth noting, though, that even 1.1 percent of a billion dollars is $11 million—a very large sum of money.

We cannot be sure that the top 400 give as much as a percentage of their annual income as some Americans of more modest means—a core point made by author Stern. But we do know, from IRS data, that the top 5 percent of all earners (with incomes of more than $200,000) earned a total of $2.2 trillion in reported adjusted gross income and donated $69 billion, or 3.1 percent of their income. This is the same figure that Stern cites for the bottom 20 percent. (It is true that those earning $50,000 and under, who earned some $514 billion, donated 4.5 percent of their income, or $23.8 billion. And the actual extent of giving by this group is likely understated, since IRS data only exist on those who itemize.)

Members of the Forbes 400 classified as “self-made” are the most generous. Forbes categorizes 279 billionaires as “self-made.” The group includes business founders, co-founders, and CEOs. The remaining 121 billionaires inherited large sums of family money or stakes in companies, which is not to say that their current wealth was inevitable. In other words, there are more than twice as many self-made U.S. billionaires as those whose wealth is based on inheritance. Self-made billionaires appear to be more generous. On average, self-made billionaires (for whom we had data) donated $83.4 billion, or approximately 6 percent of their net worth. In comparison, billionaires who had inherited significant wealth donated $12.9 billion, or approximately 3.7 percent of net worth. It is important to note that all these figures are minimum approximations, since data on many donations (for all those in the 400) are not publicly available.

It is worth reflecting on the disparity between the philanthropic habits of the self-made and those who inherited some of their wealth. In effect, the self-made can be said to be somewhat more inclined to help others climb as they have. (See Mark Zuckerberg, who attended public high school in suburban New York for two years—and went on to donate $100 million to the Newark public schools.) In other words, they have not forgotten where they came from.

Education is the most favored philanthropic cause, but there is great variety—even idiosyncrasy—in what is supported. It goes almost without saying that Americans believe that education is the key to upward mobility and to economic growth broadly. Thus it is not surprising to see that the Gates Foundation and members of the Walton family have supported K–12 education. Nor is it a surprise to see philanthropic investments in scientific and medical research (Eli Broad and MITs Broad Institute) or great museums (Charles Koch and New Yorks Museum of Modern Art).

But the virtues of philanthropy include its capacity to support the original and the unusual, the sorts of causes that are unlikely to attract government funding. For instance, Larry Page and Eric Schmidt of Google have supported the B612 Foundation, dedicated to seeking ways to divert the path of potentially destructive asteroids that might strike the Earth (named for the fictional asteroid on which the hero of Antoine de Saint-Exupérys “Little Prince” lived). The project has roots in NASA but is providing funding that NASA could not obtain. (After the recent meteor hit Siberia, interest in the foundation spiked.)

That sort of unconventional funding—what I call “independent philanthropy,” in my forthcoming book for Encounter, Philanthropy Under Fire—may be the most important reason to appreciate the philanthropy of the Forbes 400. Indeed, we might think of their $300 billion-plus in pledges as a national philanthropic venture fund, directed by people who are often active in for-profit venture finance and start-ups. One may believe that the Forbes 400 should be giving more, but they assuredly have given and pledged a significant portion of their wealth to this point.

There is reason to think, as well, that the philanthropy of the U.S. superrich is influencing international norms. Since the advent of the Giving Pledge by Bill Gates and Warren Buffett (who have pledged their entire fortunes to the Bill and Melinda Gates Foundation), several non-U.S. billionaires have made large pledges, as well. This hardly seems like a coincidence. Hong Kong shipping tycoon Li Ka-shing, for instance, has pledged a third of his $31 billion fortune to education, health care, arts, culture, and humanitarian causes.

The American philanthropic tradition is a deep one—predating the advent of the income tax. Indeed, Benjamin Franklin, who would go on to create a trust that led to the establishment of Philadelphias science museum and a vocational training school in Boston, once wrote, presaging Gates and Buffett, that “I would rather have it said, He lived usefully than He died rich.”

One cannot say with any certainty that changes in the tax code will turn off that flow of philanthropic dollars. But we should consider carefully whether it is worth taking that chance.